Yes, I think you probably are. The expensive appliances to run are the ones that produce heat. If you're using these a lot during the day, could account for it, though you'd need to be using them a hell of a lot if that's what you pay a month. The storage heaters should be run during the night, on a lower tariff than the normal domestic rate, but are still probably the most expensive form of central heating as well as being the crappiest, for lots of different reasons. If you're using some form of meter payment, the landlord (if you have one) may have some control over it.